We applied the transgenic mouse for analysis of tritium beta rays-induced mutation. In prsent study. Ingeno mouse developped by Gossen was used to detect the deleted mutation by ionizing radiation. Although coventional transgenic mouse is not appropriate for study of radiation-induced mutation, Ingeno mouse is expected to be useful for such mutations by transfection of 40 copy LacZ gene with plasmid pUR228. However, there are several problems to overcome for practical utilization. Among them, we successfully improved here technical problems, such us rescure methods of mutated lacZ gene into E.Coli and also positive selection of mutated gene by p-gal, indicating the availability for tritium biological study. This is confirmed by the evidences that RBE of neutron and carbon beam assayd by using Ingeno mouse is in agreement with that reported previously by others. These results also suggest that RBE of tritium is approximately 2. However, Ingeno mouse was appeared to be less radiation sensitive, i.e., 3.22*10^<-4>,5.37*10^<-4>,4.68*10^<-4> of mutation freguency at 0.5Gy, 10Gy of irradiation dose, respectively, and show necessity of further improvement as to radiation sensitivity. Using the hamster hybrid cells containing a single human X chromosome, we clarified the reason for less sensitivity of Ingeno mouse, suggesting the possibility of develop for radiation sensitive new transgenic mouse, which is 100-fold more sensitive than conventional assay. Consequently, our results demonstrated the usefulness of trasgenic mouse for future tritium biological study, such as specificity of DNA damage or hormesis response by tritium.